In conventional radio communication, an acknowledgement (ACK) signal is used to maintain the reliability of data transmission. When a module transmits data to another module, the ACK signal is generally sent to the module at the transmitting end. The ACK signal only indicates that the module at the receiving end has received the data properly.
As an example of conventional techniques, Japanese Patent Application laid open No. S63-173437 has disclosed a “Multiple Address Communication System”. In the multiple address communication system, a function for collecting the ACK signals is distributed to one or more branch nodes in a communication tree, which participate in broadcasting. Having received the ACK signals from all its downstream nodes in the communication tree, each of the branch nodes transmits the next message.
As another example of conventional techniques, Japanese Patent Application laid open No. H06-103482 has disclosed a “Data Collecting Device”. The data collecting device accurately collects the data of physical quantity detected by respective sensors of sensor devices, each having a communication function, distributed and arranged in specific areas at a center unit by radio waves with small power.
In a sensor network intended for an outdoor setting to collect data (e.g. environmental data) as in the conventional techniques exemplified above, each module has no power supply from outside and works on a solar battery. Consequently, there is little chance that the module receives a sufficient supply of power. For this reason, it has been required to reduce electric power consumption by the module.